1. Field of the Invention
The present invention relates to a process for recovery of nutrients from a wastewater. As used through out this specification, the term “wastewater” is intended to any wastewater contain ammonia (also referred to in this specification as “ammonium species”) and phosphorus. Non-limiting examples of wastewaters which may be treated using the present process include water borne waste flow of human domestic, industrial, commercial or agricultural origin.
2. Description of the Prior Art
The need to protect natural waters from excessive concentrations of nitrogen (N) and phosphorous (P) is well documented and widely accepted. Most jurisdictions in developed countries regulate the concentration of N and P which can be released into receiving waters.
The prior art includes the development of numerous technological advances which purportedly effect these controls.
Generally, the prior art advances fall into the categories of: (i) diversion for land application (DFLA) for use as fertilizer, or (ii) conversion to an innocuous form. For anthropogenic wastes, DFLA is only practiced in a few instances in developed countries using urine from specialized waste-separating toilets. DFLA is the primary choice for disposal of agricultural wastes. When dealing with liquid wastes from anthropogenic sources, soluble N is biologically converted to nitrogen gas through the processes of nitrification and denitrification (NDN); and P is generally converted by complexing with an aluminum or iron salt to form an insoluble and biologically unavailable precipitate. Other rarely used, but available solutions are: a commercial process for recovering P as calcium phosphate, adjusting the pH and dosing the wastewater containing ammonia with phosphorous and magnesium to cause struvite precipitation, and raising the pH and stripping the ammonia in a column.
Despite the advances made in the art, there is still room for improvement. For example, the disadvantages of the current state of the art lie with the considerable volume and energy requirements which are inherent in the processes. Specifically, DFLA requires large storage volumes for the resultant fertilizer products since it is only possible to make agricultural applications one or two times per year. Further, this approach is accompanied by the release of undesirable odors; both from the storage facilities, and when the wastes are applied to the land. Still, further depending on temperature and soil moisture, a considerable fraction of the applied ammonia may be lost by volatilization.
DFLA is energy intensive as a form of fertilizer application in that it is a separate process and is not combined with other tillage activities. DFLA does not allow the control of the fertilizer composition and its application must be regulated to prevent overloading of the soils with particular constituents (e.g., P). Further, DFLA often involves considerable transport costs when relatively dilute materials are transported long distances. This often leads to over application, and consequential problems with overland runoff. This is estimated to contribute as much as two-thirds of the surface water pollution in the USA.
Conversion of ammonia by NDN requires large volumes in the treatment plants, considerable expenditure of energy for aeration, and more elaborate process control over and above that required for simple organics removal. In many instances an electron acceptor such as methanol must be added at extra expense to facilitate denitrification. Additionally a valuable fertilizer is destroyed rather than being recovered for reuse.
P sequestering generally results in considerable additional sludge being generated when metal salts are added due to co-precipitation of other naturally occurring species in addition to the P. This sludge must be treated and then transported to a disposal site. A potentially useful mineral is turned into a waste product rather than being diverted for use as fertilizer.
Thus, despite the advances made to date, there is still considerable room for improvement.